PSI - Issue 19

Wolfgang Schneller et al. / Procedia Structural Integrity 19 (2019) 556–565 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 6. Failure origin of one unprocessed specimen

4. Conclusions The results presented in this work reveal a significant impact of microstructural features, residual stresses and surface characteristics on the fatigue strength. The microstructure of the HIP and SA condition is positively effected in terms of resistance against crack initiation and propagation by Fe-rich precipitates and Si-agglomerations, which are formed within the microstructure due to the heat input of the post treatment. Additionally, a decrease in porosity is observed for both conditions, which also enhances fatigue properties. Furthermore, the surface roughness is found to be mitigated for the post treated conditions which also leads to an improved fatigue behavior. Lastly, the measured axial surface residual stresses are lower for the HIP and SA condition than for the AB condition, which also may contribute to a further enhancement of fatigue properties. Concluding, both post treatments positively affect the presented influencing factors which lead to an improvement regarding fatigue. Comparing the machined condition, HIPing increases the fatigue strength by +13.8% and SA by +5.9% referring to the AB condition. For the unprocessed test series, the post treatments improve the fatigue strength by +25.3% for both test series. The given values are evaluated for a survival probability of 50%. It is shown that for all test series, which exhibit the as-built surface, the failure initiates from the surface due to the roughness topography along with present comparably high axial tensile residual stresses. This causes a significant reduction of cyclic material properties of approximately -60%. The outcome of this investigation is that the fatigue strength can be improved by a proper post treatment due to beneficial effects on decisive influencing factors. Furthermore, a significant reduction of the fatigue limit is caused by the as-built surface.